1. Field of the Invention
The present invention relates to an electrical apparatus suitable for installation in prefabricated installations inside a metal or insulating casing, which form medium-voltage switchboards. In particular, it relates to an apparatus which, in the closed position, is able to convey in a controlled manner, in keeping within the limits set forth in relevant regulations, the rated current required by the load to which the apparatus is connected and withstand the short-circuit current which may arise as a result of a fault at its terminals. The apparatus according to the invention is able to establish, in its open position, the short-circuit current due to a fault at its terminals. Finally, in the open position, it is able to ensure a disconnecting level suitable for the voltage level of the installation, again in accordance with the regulations.
2. Related Art and Other Considerations
Various apparatuses of the type comprising a switch and a disconnector for electric substations are known. For example, with regard to medium voltage, switch and disconnector apparatuses which perform switching and disconnecting functions in air, gas (for example SF6), oil or a vacuum are known.
Within the category of air switch and disconnector apparatuses, three main types of apparatuses are known, said apparatuses differing from each other on the basis of the type of movement of the movable contacts with respect to the fixed contacts. The first type consists of the rotating type with a central axis of rotation, the second type consists of the linear movement type and the third type consists of the hinge type.
In rotating switch and disconnector apparatuses with a central axis of rotation the movable contacts are in the form of contact blades which extend diametrically from a central axis of rotation about which they are able to rotate. In a first position of rotation, the movable contacts are in contact with the fixed contacts, while in a second position the movable contacts and the fixed contacts are at an isolating distance. In rotating hinge-type switch and disconnector apparatuses, an axis of rotation is formed on the lower contact, about which the movable contact rotates, said contact in a first position being in contact with the upper fixed contact and in a second position being situated at an isolating distance therefrom. In linear switch and disconnector apparatuses there is an upper fixed contact and a lower fixed contact of the sliding type. A conducting tube or bar is moved vertically with a translatory movement, while maintaining the electrical contact between the busbars (or the tube) and the lower contact. The apparatus is able to assume two positions: a first position in which the movable conducting tube or bar is inserted inside the upper fixed contact and a second position in which it is situated at an isolating distance therefrom.
Usually, whatever the type of air switch and disconnector apparatus, an open switching chamber is provided, inside which circuit breaking of the load current is performed by means of a small air jet produced by a piston, the movement of which is guided directly by the displacement of the contact blades of the switch and disconnector apparatus.
The rotating hinge-type switch and disconnector apparatuses are commonly used on overhead supply lines as disconnecting apparatuses and in exposed switchboards (in so-called “raised substations” or “tower substations”). These apparatuses, however, are not suitable for installation in prefabricated installations inside a metal casing (switchboards formed by means of prefabricated cubicles) since in this case segregation between the two terminals, i.e. the upper and lower terminal of the apparatus, is required, at least when the switch and disconnector apparatus is in a open condition, so as to allow access to the cable bay of the cubicle with the live busbars. This latter condition is difficult to achieve in the case of hinge-type switch and disconnector apparatuses. With the advent of prefabricated switchboards and with the introduction of new safety standards, switch and disconnector apparatuses of the rotating type and switch and disconnector apparatuses of the linear type which are more suitable for installation in prefabricated switchboards have been developed.
The purpose of switch and disconnector apparatuses and any device present in a network node is to allow switching and disconnecting of the electric lines which extend from the network node in question. Disconnection is required in particular to allow the operators to be able to carry out maintenance along the whole length of the disconnected line in conditions of maximum safety and with the other lines connected to the network node in question remaining in the live condition. The advent of prefabricated switchboards inside a metal casing has made it possible to achieve the above with a greater degree of safety, namely allow full access to the line which extends from the network node, to which node it is connected via the switch and disconnector apparatus. It is therefore necessary for the operator to be able to access the cubicle connected to the line in question in conditions of maximum safety. When the operator accesses the cubicle, all the parts which are normally live must, in accordance with the regulations, be disconnected and earthed; this is achieved by means of the said switch and disconnector apparatus and an earthing disconnector. This earthing disconnector is interlocked with the door providing access to the cubicle so that it is not possible to open said door if the earthing disconnector is not in the closed position. As mentioned above, moreover, it is necessary to ensure that this line, connected to the cubicle in question, is safely disconnected when the operator accesses the inside of the cubicle. This condition is guaranteed by means of a system mechanically interlocking the switch and disconnector apparatus and the earthing disconnector, the interlocking system preventing closing of the earthing disconnector if the switch and disconnector apparatus is in the closed position, and vice versa. Finally, it is necessary to ensure adequate electrical safety conditions for the operator entering the cubicle, preventing direct contact With the busbars of the switchboard which are still live. For cost-related and dimensional reasons generally the tendency is to use switch and disconnector apparatuses also in order to perform segregation between the busbar bay (inside which the live busbars are present) and the line cell (accessed by the operator). According to Italian patent application MI95A002592, this segregation is performed by means of a metal gate which is arranged between the fixed contacts and the movable contacts of the switch and disconnector apparatus when the latter is isolated. The displacement of the metal gate is associated with the movement of the earthing disconnector, in particular closing of the earthing disconnector also closes simultaneously the metal gate so that, when the operator opens the door of the cubicle for access thereto, in addition to a guarantee that the normally live parts are earthed (by means of the gate), there is also the guarantee of segregation from the bay containing the busbars which are still live. The presence of the shutter gate is therefore of fundamental importance when the substation is open and personnel have access thereto.
The presence of the shutter gate ensures that the apparatus complies with the safety conditions according to the regulations. Nevertheless, the Applicants have established that there is need to provide an even greater degree of protection.
Based on the fact that all additional devices for performing a function may intrinsically be the cause of a malfunction, the Applicant has set itself the aim of producing a similar segregating function without any additional mechanical device (such as the metal gate and its actuating arrangement in the known apparatus according to Italian patent application MI95A002592).
In addition to this, it is known that the switching (circuit breaking) performance of all the air switch and disconnector apparatuses is greatly dependent upon the environmental conditions, in particular the humidity of the air and the presence of polluting substances or salinity (in environments close to the sea). For example, in environments which are polluted or have a severe climate there is a deterioration of the conductive materials, in particular the parts making sliding contact, until, with time, seizing of said parts occurs.
It must be remembered that the switch and disconnector apparatuses, in most applications, may be subject to periods during which a high number of operations is required and periods of inactivity, but the fundamental condition is that when these apparatuses are required to operate they must do so in an absolutely safe and reliable manner. This is possible only if the contacts of the apparatus are kept in working order and clean. However, in the case of installations in humid and/or polluting climates, this is not possible with the known linear air switch and disconnector apparatuses unless they are subject to frequent programmed maintenance. On the other hand, the present tendency, primarily for cost-related reasons, is that of keeping the maintenance carried out on apparatus to a minimum. It is therefore clear that there exists the need to provide an apparatus which is segregated as far as possible from the external environment and therefore not affected by the conditions prevailing therein.
Another drawback precisely of linear air switch and disconnector apparatuses is that they must be mounted on a particularly strong metal structure which is able to withstand, without deformation, considerable forces so as to operate the arm which moves axially the bar or tube of conductive material.
Frequently the switch and disconnector apparatuses are mounted in sets of two or three so as to obtain, respectively, two-phase or three-phase apparatuses. With regard to two-phase or three-phase linear air switch and disconnector apparatuses, there is no segregation of the various phases. Therefore, in known switch and disconnector apparatuses there is the risk that an arc may occur between phases and that an (otherwise harmless) “non interruption” may result in a two-phase or three-phase short-circuit. The only precaution which is adopted in double-pole or triple-pole linear air switch and disconnector apparatuses is that of arranging an individual switch and disconnector apparatus at a suitable distance from the next one. This does not eliminate entirely the abovementioned problems and in any case results in the apparatus being excessively voluminous.
FR 870 991 A discloses an electric switch suitable for being used in environments exposed to explosive gas.
DE 957 410 C discloses a single-pole off-load switch.
DE 73 16 028 U discloses an insulate-totally enclosed high voltage switching station.
BE 788 180 A1 discloses a supply disconnection element for an electric apparatus.
U.S. Pat. No. 3,259,726 A discloses a telescoping type circuit breaker having isolator contact with arc snuffing means.
U.S. Pat. No. 3,814,883 A discloses a gas-blast circuit interrupter with insulating arc shield.
In the light of the problems and the limitations of known switch and disconnector apparatuses, the Applicant has established that there is a need to provide an improved linear air switch and disconnector apparatus which is able to solve said problems and overcome said limitations.
Therefore, the main object of the present invention is to provide a linear air switch and disconnector apparatus which is safe and reliable and in which the circuit breaking/switching performance is not substantially dependent upon the environmental conditions (within a protected atmosphere).
A further object of the present invention is to provide a linear air switch and disconnector apparatus which is more compact than the existing similar apparatuses, does not require a particularly strong support structure and is sufficiently low-cost.
A further object of the present invention is to provide a linear air switch and disconnector apparatus having each phase segregated so that there is absolutely no risk of a “non-interruption” of the apparatus resulting in a two-phase or three-phase short-circuit.
These and other objects are achieved by means of a linear air switch and disconnector apparatus having the characteristic features of the independent claim 1. Further advantageous features of the present invention are contained in the dependent claims. All the claims are considered to form an integral part of the present description.